An electronic image sensor captures images using light-sensitive photodetectors that convert incident light into electrical signals. Image sensors are generally classified as either front-illuminated image sensors or back-illuminated image sensors. FIG. 1 is a simplified illustration of a front-illuminated image sensor in accordance with the prior art. Image sensor 100 includes pixels 102, 104, 106 formed within a sensor layer 108 and a circuit layer 110. Photodetectors 112, 114, 116 are formed in sensor layer 108. Conductive interconnects 118, 120, 122, such as gates and connectors, are formed in circuit layer 110.
Unfortunately, the positioning of conductive interconnects 118, 120, 122, and various other features associated with circuit layer 110, over photodetectors 112, 114, 116 adversely impacts the fill factor and quantum efficiency of image sensor 100. This is because light 124 from a subject scene must pass through circuit layer 110 before it is detected by photodetectors 112, 114, 116.
A back-illuminated image sensor addresses the fill factor and quantum efficiency issues by constructing the image sensor such that the light from a subject scene is incident on a backside of a sensor layer. The “frontside” 126 of sensor layer 108 is conventionally known as the side of sensor layer 108 that abuts circuit layer 110, while the “backside” 128 is the side of sensor layer 108 that opposes frontside 126. FIG. 2 is a simplified illustration of a back-illuminated image sensor in accordance with the prior art. Circuit layer 110 is positioned between support substrate 202 and sensor layer 108. This allows light 124 to strike the backside 128 of sensor layer 108, where it is detected by photodetectors 112, 114, 116. The detection of light 124 by photodetectors 112, 114, 116 is no longer impacted by the metallization level interconnects and other features of circuit layer 110.
Back-illuminated image sensors, however, can present a new set of challenges. Interface 204 between sensor layer 108 and insulating layer 206 can produce high levels of dark current and a loss of quantum efficiency, especially in the blue light spectrum. This is due to the presence of dangling bonds at the etched silicon surface of backside 128. Moreover, conventional passivation techniques for passivating interface 204 can be adversely impacted by subsequent processing steps during fabrication of image sensor 200.
FIG. 3 depicts an exemplary doping profile of interface 204 along line A-A′ in FIG. 2. Conventional back-illuminated image sensors are constructed as n-type metal-oxide-semiconductor (NMOS) image sensor. Thus, the n-doped photodetectors are formed in a well or layer doped with one or more p-type dopants. Line 300 depicts a doping profile of boron dopants (p-type) at interface 204 prior to the performance of subsequent Complementary Metal Oxide Semiconductor (CMOS) fabrication steps on image sensor 200, while line 302 illustrates the doping profile of boron at interface 204 after the performance of the subsequent CMOS fabrication steps. As shown in FIG. 3, the boron dopants diffuse out of sensor layer 108 and into insulating layer 206 during the subsequent CMOS fabrication steps. This diffusion creates a drop in doping profile 304 on the sensor layer side of interface 204. The drop in the doping profile produces an unwanted electrostatic potential well that traps photo-induced charge carriers at interface 204. Substituting slower diffusing p-type dopants, such as indium, for boron can reduce the thermal diffusion during processing, but indium increases the number of dark field bright point defects in the image sensor.
Accordingly, a need exists for improved processing techniques for forming back-illuminated image sensors.